The present invention relates to the field of exhaust gas treatment.
Recently, in order to reduce fuel consumption, the motor vehicle industry has developed gasoline engines that operate with lean air/fuel mixtures. An air/fuel mixture is considered to be lean when it contains more oxygen than would be required for complete combustion of the fuel. By contrast, in a rich air/fuel mixture, there is less oxygen than is required for complete combustion of the fuel. Engines that run on lean air/fuel mixtures are referred to as lean-burn engines.
For the quantitative representation of the relationship of the amount of air to fuel that is supplied to an engine, the air/fuel ratio or normalized air/fuel ratio lambda (xcex) is used. The air/fuel ratio indicates how many kilograms of air per kilogram of fuel are supplied to the engine. For stoichiometric combustion, the air/fuel ratio for conventional engine fuels is 14.7. The associated normalized air/fuel ratio xcex is the air/fuel ratio normalized to stoichiometric conditions, and therefore has the value 1.0 for a stoichiometrically composed air/fuel mixture. The air/fuel ratio of the exhaust gas leaving the engine is the same as the air/fuel ratio of the fuel supplied to the engine when no selective storage processes for specific exhaust gas components occur in the engine.
Lean-burn engines run with air/fuel ratios corresponding to normalized air/fuel ratios of more than 1.3 under normal operation. The oxygen content of the exhaust gas from these engines is usually between 3 vol. % and 15 vol. %. When accelerating and also under full load, stoichiometric, or slightly rich air/fuel mixtures are also supplied to lean-burn engines in order to be able to provide the high power required.
The treatment of exhaust gases from lean-burn engines causes considerable problems. When operating lean-burn engines, nitrogen oxides present in the exhaust gas under lean operation cannot be reduced via a chemical route due to the high oxygen concentration in the exhaust gas. Usually, from about 60 vol. % to about 95 vol. % of the nitrogen oxides present in the exhaust gas consist of nitrogen monoxide, depending on the operating status of the engine. The remainder is made up of nitrogen dioxide and other nitrogen oxides.
To solve the exhaust gas problems associated with lean-burn engines, it has been suggested that the nitrogen oxides be converted to nitrogen and water using the principle of selective catalytic reduction, which makes use of ammonia and the oxygen present in lean exhaust gases, on a suitable catalyst, such as an SCR catalyst. This type of catalyst contains, for example zeolites, silicon dioxide, aluminium oxide and/or titanium oxide as support material for copper, iron, platinum, palladium and/or rhodium, vanadium and/or tungsten and also always has a certain storage capacity for ammonia.
It is also known in the art that in order to improve the reduction of nitrogen oxides in lean exhaust gases from internal combustion engines, the concentration of nitrogen dioxide in the exhaust gas may first be increased to 50 vol. %. This knowledge is described in DE 198 20 682 A1, which is incorporated by reference herein. The concentration is increased by oxidation of nitrogen monoxide to nitrogen dioxide in an electric gas discharge. The treated exhaust gas is then passed over an SCR catalyst, while a reducing agent, preferably ammonia, is supplied. Catalysts known from denitrifying the waste gas of power stations and also copper cation exchanged ZSM-5 zeolites are mentioned as suitable SCR catalysts.
It has also been proposed that the ammonia required for the SCR reaction be synthesized on board a motor vehicle from constituents of the exhaust gas. This proposal was described in EP 0 773 354 A1, which is incorporated by reference herein. For this purpose, the exhaust gas is passed over a three-way catalyst and then over an SCR catalyst, and the engine is operated alternately with lean and rich air/fuel mixtures. During operating phases with rich air/fuel mixtures, the three-way catalyst forms ammonia from the nitrogen oxides present in the exhaust gas, which is temporarily stored on the SCR catalyst. During operating phases with lean air/fuel mixtures, the nitrogen oxides present in the exhaust gas pass through the three-way catalyst virtually unchanged and are reduced to nitrogen and water by the ammonia adsorbed on the SCR catalyst.
A further development of this process has also been proposed. In this process, a third catalyst is introduced in the exhaust gas stream, upstream of the other two catalysts. This method is described in DE 198 20 828 A1, which is incorporated by reference herein. This third catalyst temporarily stores the nitrogen oxides present in the exhaust gas in the presence of a lean exhaust gas composition and releases it again in the presence of a rich exhaust gas composition. Due to the use of the much larger amounts of temporarily stored nitrogen oxides produced under lean operation than under rich operation, a correspondingly large amount of ammonia can be produced and temporarily stored in a particular rich combustion management phase. This provides for effective nitrogen oxide reduction in a subsequent lean operating phase. Overall, this should enable operation with a high proportion of lean phases and as a result with correspondingly low fuel consumption.
Although the aforementioned proposals lead to improved conversion of the nitrogen oxides emitted by lean-burn engines, a further improvement in nitrogen oxide conversion is required to comply with future exhaust gas standards. The object of the present invention is to provide an exhaust gas treatment unit that enables a further improvement in nitrogen oxide conversion in the exhaust gas from lean-burn engines.
The present invention provides an exhaust gas treatment unit for internal combustion engines. A first catalyst unit produces ammonia from relevant exhaust gas constituents of a rich exhaust gas composition. A second catalyst unit, located downstream of the first catalyst unit, temporarily stores the ammonia produced by the first catalyst unit and under lean exhaust conditions, enables nitrogen oxides (NOx) present in the exhaust gas to undergo a reduction reaction using the temporarily stored ammonia as a reducing agent. Prior to introduction of the lean exhaust gas to the second catalyst unit, it is exposed to a third catalyst unit in which a portion of the nitrogen oxides is oxidized.
Thus, in one embodiment, the present invention provides an exhaust gas treatment unit for an internal combustion engine. The exhaust gas treatment unit comprises:
a. a first catalyst unit, wherein said first catalyst unit comprises a three-way catalyst;
b. a second catalyst unit, wherein said second catalyst unit is located downstream of the first catalyst unit and comprises an SCR-catalyst; and
c. a third catalyst unit, wherein said third catalyst unit is located downstream of said first catalyst unit and upstream of said second catalyst unit, and said third catalyst unit is capable of oxidizing the nitrogen oxides present in the exhaust gas at lean exhaust gas conditions so that from 25 to 75 vol. % of the nitrogen oxides entering the second catalyst unit are nitrogen dioxide.
The present invention also provides a process that provides for an improved conversion of nitrogen oxides. Under one embodiment, the process comprises:
a. exposing a rich exhaust gas to a first catalyst unit to generate ammonia;
b. storing said ammonia of step (a) in a second catalyst unit, wherein said second catalyst unit is located downstream of said first catalyst unit;
c. exposing a lean exhaust gas to a third catalyst unit, wherein said third catalyst unit is located downstream of said first catalyst unit and upstream of said second catalyst unit, and wherein said lean exhaust gas comprises nitrogen oxides;
d. oxidizing said nitrogen oxides to form oxidized nitrogen oxides; and
e. exposing said oxidized nitrogen oxides to said second catalyst unit.
For a better understanding of the present invention together with other and further advantages and embodiments, reference is made to the following description taken in conjunction with the examples, the scope of which is set forth in the appended claims.